Men and non-pregnant women non-smokers of all ethnic backgrounds between the ages of 18 to 50 who are free of major medical illness and who agree and are medically able to abstain from alcohol and all drugs, to adhere to a regular sleep schedule, and to limit caffeine-intake to less than or equal to 2 cups of coffee per day for at least two weeks (prescription drugs, 4 weeks) before, and for 4 weeks during the treatment period are eligible to participate.